The invention relates to a progressive spectacle lens as set out in the preamble of claim 1, the lens producing only slight dynamic distortion.
Progressive spectacle lenses (also called varifocal lenses, multifocal lenses etc.) are usually understood to be spectacle lenses having a different (lower) power in the region through which a spectacles wearer views an object located at a great distancexe2x80x94hereunder referred to as a distance portionxe2x80x94than in the region (near portion) through which the spectacles wearer views a near object. Located between the distance portion and the near portion is the so-called progressive zone in which the power of the spectacle lens continuously increases from that of the distance portion to that of the near portion. The magnitude of the power increase is also designated as addition power.
As a rule, the distance portion is located in the upper part of the spectacle lens and is designed for viewing xe2x80x9cto infinityxe2x80x9d, whilst the near portion is located in the lower region and is particularly designed for reading. In spectacles for special applicationsxe2x80x94those for pilots or for monitor work stations are mentioned as examplesxe2x80x94the distance and near portions may also be arranged differently and/or designed for other distances. Furthermore, it is possible for a plurality of near portions and/or distance portions and suitable progressive zones to be present.
With progressive spectacle lenses having a constant refractive index it is necessary, in order that the power may increase between the distance portion and the near portion, that the curvature of one or both surfaces continuously change from the distance portion to the near portion.
The surfaces of spectacle lenses are usually characterized by the so-called principal radii of curvature R1 and R2 at every point on the surface. (Sometimes also the so-called principal curvatures K1=1/R1 and K2=1/R2 are given instead of the principal radii of curvature.) Together with the refractive index n of the glass material, the principal radii of curvature govern the parameters frequently used for an ophthalmologic characterization of a surface:
Surface power=0.5xc2x7(nxe2x88x921)xc2x7(1/R1+1/R2) 
Surface astigmatism=(nxe2x88x921)xc2x7(1/R1xe2x88x921/R2). 
Surface power is the parameter via which an increase of power from the distance portion to the near portion is achieved. Surface astigmatism (more clearly termed cylinder power) is a xe2x80x9ctroublesome propertyxe2x80x9d, because an astigmatismxe2x80x94inasmuch as an eye does not have an innate astigmatism to be correctedxe2x80x94which exceeds a value of about 0.5 dpt results in an indistinctly perceived image on the retina.
Although any change of the curvature of the surface which is needed to achieve a surface power increase without vision being xe2x80x9cdisturbedxe2x80x9d by surface astigmatism can be attained relatively simply along a (plane or winding) line, considerable xe2x80x9cintersectionsxe2x80x9d of surfaces will result alongside this line, leading to a large surface astigmatism which more or less impairs the lens in regions alongside the mentioned line.
Furthermore, the strong variation of the prismatic powers results in dynamic distortion effects, the cause of which will be explained in the following:
For this, the image of a group G of n pairs of different object points P(x, y) which are located in a plane at a distance s in front of the spectacle lens will be considered. Without limiting the generality, these shall be disposed in the form of a grid having equal spacings. If these object points P(x, y) are imaged by a spectacle lens in such manner that the principal rays pass through a point Z located on the eye side (for example, the center of rotation of the eye or the entrance pupil of the eye), and if the eye-side principal rays intersect a second plane located at a distance r from the spectacle lens, in the following referred to as a projection plane, then a second group B of points results which are the image points of the object points P(x, y) in the projection plane.
Generally stated, the spectacle lens performs an imaging A from the object plane onto the projection plane in such manner that
A:GB 
As a rule, the imaged grid is no longer equally spaced, but warped because of a known image defect which is termed xe2x80x9cdistortionxe2x80x9d. When a back-side aperture is present in a path of rays, as is the case with a spectacles wearer, the distortion is cushion-shaped with a single vision positive lens and barrel-shaped with a single vision negative lens. With progressive lenses mixed forms may arise.
According to the invention it has been realized that additional effects arise when a temporal change of the distorted grid {right arrow over (xcexd)}B in the projection plane with translational movements of the object grid {right arrow over (xcexd)}G are considered. The movement of the grids are represented in a usual manner by vector fields for the velocity {right arrow over (xcexd)}(x,y). The grid point having the subscript i has the velocity {right arrow over (xcexd)}(xi,yi).
It must be remarked that within a particular simply configured region each point P(x, y) in the object plane is imaged onto the projection plane. The limitation to a countable finite number of discrete grid points Pi is made only for the sake of graphical clarity. Thus, n may be finite or infinitely large.
When a human eye is confronted with an internally rigid, but moving object grid which has been imaged by a progressive lens, then during the movement the distorted grid will appear to move in the projection plane not only rigidly as a whole. Rather than this, in addition to the expected directed movement caused by the translational movement of the object grid, an undirected component will be observed
{right arrow over (xcexd)}B={right arrow over (xcexd)}BDirected+{right arrow over (xcexd)}BUndirected. 
In general, the vector field {right arrow over (xcexd)}B is free neither from divergence nor rotation:
There will be regions in which the density of the grid points increases during the movement ∇xc2x7{right arrow over (xcexd)} greater than 0, and others in which it decreases ∇xc2x7{right arrow over (xcexd)} less than 0: the grid will appear to be subjected to a kneading operation.
For rotating movements of glance the following also applies: ∇xc3x97{right arrow over (xcexd)}xe2x89xa00.
The dynamic effects are particularly striking when the directed component is subtracted from the total velocity field. All statements made in the present application concerning track curves or trajectories relate to so-called trajectories in a coordinate system of a particular reference point which is the stationary point.
It is expedient to select this marked point to be close to the center of the lens; within the scope of this application and without prejudice to generality, it has the coordinates (0, 0) in the used Cartesian coordinate system which has its origin at the object-side vertex of the spectacle lens. The z axis is directed along the direction of light. The surface-perpendicular vectors of the mentioned planes have only one z component.
To describe the movement of the object points arranged in the form of an equally spaced grid, in the following the trajectories of the undirected components of the grid points during a horizontal periodic movement of the object grid will be considered. The track curve of an arbitrarily chosen object point will then be a horizontal straight line, and that of the conjugated image point will be a curve lying in a plane. This curve is characterized by the variation of the prismatic power (or the prismatic secondary effect) along the principal rays generating the curve.
The horizontal movements of glance discussed here frequently occur, for example during reading, when driving a car, or when working with a computer.
For a single-vision lens having vanishing power the undirected component is equal to zero. The trajectories degenerate to points. The condition of the grid is stationary.
In the general case of non-vanishing power, single-vision lenses have the characteristics shown in FIG. 1. In the left-hand partial illustration the trajectories of a horizontal movement are illustrated for a negative lens having a spherical power of xe2x88x922.5 dpt, and in the right-hand partial illustration for a positive lens having a spherical power of +2.5 dpt.
The stationary point at the center, to which the undirected velocity component of the imaged grid is referred, can be seen.
On the right and left of this stationary point are purely horizontal tracks having a vanishing vertical component: with a purely horizontal movement of glance only the horizontal component of the prismatic secondary effects varies along the trajectories in this region; the vertical componentxe2x80x94and with it the slope of the track curvesxe2x80x94is equal to zero.
The length of the plotted trajectories monotonously increases from the stationary point outwards in the radial directionxe2x80x94as also does the prismatic secondary effect.
When moving upwards or downwards from the stationary point, an increasing curvature of the track curves may also be discerned. This results because the vertical deflection of the principal rays during a movement varies more strongly than it does further inwards. The position of the centers of curvature correlates with the distortion.
An inwardly directed opening of the curves (towards the center of the lens) signifies a barrel-shaped distortion, whilst an outwardly directed opening signifies a cushion-shaped distortion.
With progressive spectacle lenses the features of the trajectories for single vision lenses are noticeably changed by the power increase of progressive spectacle lenses.
FIGS. 2, 3, 4 and 5 show trajectories of spectacle lenses on the market; these appear to have been constructed in accordance with the following patent publications:
FIG. 2 DE-C-28 14 936
FIG. 3 WO 95/27229
FIG. 4 DE-C-43 42 234
FIG. 5 U.S. Pat. No. 4,606,622 or DE 196 12 177.
FIGS. 2 to 5 show right-hand side lenses which are on the market; these have the prescription sph +0.5 dpt, cyl 0, Add 2.0 dpt, Pr 0 (all plots were computed for r=0 mm and s=40 mm).
The trajectories of a relatively old progressive spectacle lens shown in FIG. 2 differ from those of all other shown lenses by being, in the bottom half, very short, extremely curved trajectories. A large number are even retrograde, i.e. with the horizontal movements of glance described here, many points appear to move at first with, and later counter to the direction of movement of the objects. This produces serious xe2x80x9cswaying sensationsxe2x80x9d, and the objects appear to be strongly dynamically distorted.
For computing a progressive surface in the wearing position, a wearing situation is established. This relates either to a particular user for whom the individual parameters have been specially determined in the respective wearing situation and the progressive surface has been separately computed and fabricated, or to average values as described in DIN 58 208, Part 2.
The invention is based on the object of further developing a progressive spectacle lens as set out in the preamble of patent claim 1 in such manner that the dynamic distortion which of necessity arises with progressive spectacle lenses has been minimized to the extent that it is no longer felt to be disturbing by a spectacles wearer.
An achievement of this object in accordance with the invention is set out in the patent claims.
According to the invention, to minimize a dynamic distortion, track curves (trajectories relative to a stationary point at (0, 0)) formed by connecting points of intersection of image-side principal rays passing through a center of rotation of an eye with a projection plane at a distance s from an object-side vertex of the spectacle lens, when horizontally moving objects having coordinates (xxe2x88x92dx, y, s) at a beginning of a movement and (x+dx, y, s) at an end of a movement are imaged through the progressive spectacle lens with r=0 mm, s=xe2x88x9240 mm and dx=35 mm, satisfy the following condition:
The absolute value of a difference between a minimum and a maximum y coordinate of a trajectory is smaller than a value H given in the following table:
Alternatively or additionally the arc length of the trajectory may be shorter than the value L given in the following table:
Preferably or alternatively the mean gradient of the trajectory may be smaller than the value m given in the following table:
Preferably or alternatively the maximum gradient of the trajectory may be smaller than the value M given in the following table:
Furthermore it is of advantage when preferably or alternatively the x coordinate of the center of the trajectory (half the sum of the minimum and the maximum x coordinate) is smaller than the value xz according to the following table:
Furthermore it is preferred when alternatively or additionally the yx coordinate of the center of the trajectory (half the sum of the minimum and the maximum y coordinate) is smaller than the value yz according to the following table:
In any case it is preferred that inter- or extrapolated values apply to not listed prescriptions.